Book/Dissertation / PhD Thesis

FZJ-2018-01925

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Magnetic Properties of Self-assembled Manganese Oxide and Iron Oxide Nanoparticles - Spin Structure and Composition

Sun, X. (Corresponding author)FZJ*

2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-345-7

Please use a persistent id in citations: http://hdl.handle.net/2128/19677

Abstract: Magnetic nanoparticles (NPs) have attracted much interest for decades due to their potential applications in high density data storage, spintronic devices and nanomedicine. In analogy to atoms, they can represent building blocks for NP superstructures. Belowa critical size, NPs prefer to be in a magnetic single domain state. In this case, each NP can be considered as possessing one large magnetic moment called \superspin". The superspins in self-assembled NP superstructures interact with each other usually via dipolar interactions. With the interplay of individual and collective behaviour of the NPs, novel materials with appealing magnetic and electronic properties may be fabricated. In this thesis, the magnetic properties of two different transition metal oxide NP and bulk systems, manganese oxide and iron oxide, were studied. For manganese oxides, the spin structure inside the NPs is of interest. Zero field cooled (ZFC) magnetization curves of MnO NPs usually show a low temperature peak at $\thicksim$ 25 K. No feature indicating the antiferromagnetic (AF)-to-paramagnetic (PM) phase transition of MnO was found near its bulk Néel temperature at 118 K. However, polarized neutron scattering shows the expected vanishing of the AF order parameter of MnO near 118 K. This contradiction between magnetometry and neutron scattering results can be explained assuming an AF-superparamagnetic (SPM) core with a ferrimagnetic (FiM) Mn$_{3}$O$_{4}$ shell. In order to study the magnetization dynamics of the AF-SPM core, AC-susceptibility measurements were performed. A low temperature peak is observed similar to that found in the ZFC curves. This peak shows a weak frequency dependence, which is expected for an AF system. To further investigate the origin of the low temperature peak, bulk MnO was studied as a reference. Magnetometry and polarized neutron scattering experiments were performed on MnO powder and a single crystal to be compared with the results of NPs. The phase of manganese oxide was tuned by annealing the as-prepared samples in various environments. Their magnetic properties were compared to that of the as-prepared ones. Moreover, Monte Carlo simulations were performed for MnO NPs and \bulk". In order to study the magnetic properties of self-organized NP superstructures, spherical iron oxide NPs were used. 2D ordered NP films were produced using various methods. The order of the NP superstructures was characterized using scanning electron microscopy (SEM) and grazing incidence small angle x-ray scattering (GISAXS). A weak feature near the Verwey transition of magnetite is found in ZFC/FC magnetization curves of NPs with 15 and 20nm diameters. Above T$_{N}$ of wüstite, the overall magnetic moment increases in the ZFC curves. This is due to the coupling between AF wüstite and FiM magnetite or maghemite. Moreover, a peak indicating the crossover between the blocked and the unblocked SPM or superspin glass (SSG) states of the NP superspins is observed in the ZFC curves. This peak temperature shows a strong field dependence as usually found for SPM and SSG systems. Moreover, the peak temperature shows a decrease as the NP size decreases due to the smaller energy barriers of smaller particles. In the hysteresis loops of 11 - 20nm NPs, an Exchange Bias (EB) effect and a hardening effect are observed. The iron oxide phases of the NPs were also tuned using various annealing procedures. By annealing the particles in vacuum at 318$^{\circ}$C for several hours, the ratio of wüstite and magnetite is increased. After the particles were annealed in air, they tend to be fully oxidized to maghemite.

Note: RWTH Aachen, Diss., 2018

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Contributing Institute(s):

1. Streumethoden (JCNS-2)
2. Streumethoden (PGI-4)
3. JARA-FIT (JARA-FIT)
4. JCNS-FRM-II (JCNS-FRM-II)

Research Program(s):

1. 144 - Controlling Collective States (POF3-144) (POF3-144)
2. 524 - Controlling Collective States (POF3-524) (POF3-524)
3. 6212 - Quantum Condensed Matter: Magnetism, Superconductivity (POF3-621) (POF3-621)
4. 6213 - Materials and Processes for Energy and Transport Technologies (POF3-621) (POF3-621)
5. 6G4 - Jülich Centre for Neutron Research (JCNS) (POF3-623) (POF3-623)

Experiment(s):

1. DNS: Diffuse scattering neutron time of flight spectrometer (NL6S)

Appears in the scientific report 2018

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